1. Field of the Invention
The present invention relates to thin disk lasers, and more specifically, it relates to a means for scaling the transverse area of the laser gain sample to scale the average power of laser systems based on the thin disk laser.
2. Description of Related Art
U.S. Pat. No. 5,553,088 is directed to a laser amplifying system having a solid body arranged in a laser radiation field and including a laser active material that is pumped with a pumping light source. The solid body has a cooling surface and transfers heat created therein to a solid cooling element via the cooling surface. In this manner, a temperature gradient results in the solid body in a direction towards the cooling surface. The solid cooling element forms a carrier for the solid body. The laser radiation field propagates approximately parallel to the temperature gradient in the solid body. By enabling heat to be transferred to the solid cooling element via the cooling surface, this structure enables the solid body to be pumped at a high pumping power. Further, since the laser radiation field propagates approximately parallel to the temperature gradient in the solid body, the radiation field sees the same temperature gradient in all cross-sectional areas. Thus, the temperature gradient does not lead to an adverse effect on the beam quality of the laser radiation field at high pumping power.
Although the thin-disk or active mirror laser architecture is a well known and demonstrated approach to generating laser radiation, its ability to scale to high-average power is limited by transverse amplified spontaneous emission (ASE). The thin disk is motivated as a gain element for high beam quality lasers because heat is removed from the back face of the disk. This geometry leads to a situation in which the transverse thermal gradients in the laser gain sample are substantially reduced, and even completely eliminated. This allows the possibility of energy extraction in a high quality laser beam that suffers little of no optical distortion due to transverse thermal gradients.
To scale the average power of laser systems based on this approach, one must scale the transverse area of the laser gain sample. Although this scaling approach works up to a point, eventually the deleterious effects of transverse ASE limit further scaling. The present invention specifically addresses this ASE limitation to scaling by substantially reducing the solid angle over which spontaneously emitted photons are trapped in the laser sample. It is this reduction in the trapped solid angle of spontaneously emitted photons that enables the thin disk laser to be substantially scaled in power output beyond what has been available.
It is an object of the present invention to scale thin disk lasers to obtain high average power values.
Using a thin disk laser gain element with an undoped cap layer enables lasers to be scaled to extremely high average output power values. Ordinarily, the power scaling of such thin-disk is limited by the deleterious effects of amplified spontaneous emission. By using an undoped cap layer diffusion bonded to the thin disk, the onset of amplified spontaneous emission does not occur as readily as when no cap is used, and much larger transverse size thin disks can be effectively used as laser gain elements.
In a conventional thin disk laser system, pump radiation passes through a dichroic beamsplitter and an output coupler to optically pump a thin disk of laser material. Heat generated in the laser crystal is drawn away from the crystal, in the downward direction, into a cooling block. The laser resonator is formed by a highly reflective coating on the side of the thin disk laser sample that is in contact with the cooling block, and the output coupler laser mirror that is coated to allow the pump radiation to pass through it. The thin disk geometry insures that heat will flow substantially in the downward direction in the sample and so result in no thermal gradient in a direction transverse to the laser axis.
Due to the total internal reflection of spontaneously emitted photons within the thin disk at its large face that is not contacted to anything, amplified spontaneous emission limits the transverse size of the thin disk that can be efficiently utilized in a laser system. The present invention reduces the solid angle over which spontaneously emitted photons are trapped and so allows the transverse size of the thin disk to be substantially increased before the deleterious effects of ASE become apparent. The present invention is constructed with identical elements as in the conventional thin disk laser described above, with the addition of an undoped crystal affixed to the opposite side of the thin disk laser sample that is in contact with the cooling block. The undoped crystal is near index matched to the thin disk crystal.
The invention differs from the conventional thin disk geometry due to the inclusion of the undoped cap layer that, in one embodiment, is diffusion bonded to the laser crystal. Because the surface of the thin disk that was previously not contacted to anything is now contacted to an undoped crystal that is near index matched to the thin disk crystal, spontaneously emitted photons are not trapped by total internal reflection at this face of the thin disk. Because photons that impinge on this diffusion bonded surface are not confined to the gain loaded crystal, they are not as effective as they previously were in generating ASE. In effect, the use of the undoped cap layer has transformed the thin disk from a geometry in which ASE was largely trapped within the thin disk to a geometry in which the ASE is unconfined. The unconfined geometry of the diffusion bonded sample allows scaling to much higher power level lasers than would be possible without the use of the cap layer.